In power supply (or other electronic devices) switches such as Bipolar Junction Transistors (BJTs), MOSFET and the like are needed. They mainly aim to set current ON or OFF for circuits. As the energy of inductors in the circuits cannot be cut off abruptly, at the switch instant a spike many times of the regular voltage often occurs. When the switches are located in the circuits of a higher electric power, occurrence of the spike could damage the circuit or switches. Snubber is developed to solve this problem. It aims to harness the spike or noise at the switch instant. The snubber usually includes a capacitor and a resistor coupling in parallel with a switch, and a diode is employed to allow current to pass through one way. As there is no additional control signal to affect operation of such a snubber, it also is called a passive snubber. Reference of the passive snubber can be found in U.S. Pat. No. 7,161,331 entitled “Boost converter utilizing bi-directional magnetic energy transfer of coupling inductor”. It has a primary circuit which includes a switch Q coupling in parallel with a capacitor C1 and diodes D1 and D2 to form a passive regenerative snubber. During switching of the switch Q one way flowing of the diode D1 and energy storing function of the capacitor C1 can reduce excessive fluctuation of voltage and current, and prevent the switch Q from being damaged by the spike. Another snubber example can be found in U.S. Pat. No. 6,876,556 entitled “Accelerated commutation for passive clamp isolated boost converters”. While the passive snubbers mentioned above and taught in other prior art can eliminate the spike, they are not controlled by signals and do not have synchronous characteristics. By contrast, there are also active snubbers. One of the examples is disclosed in U.S. Pat. No. 5,570,278 entitled “Clamped continuous flyback power converter”. It has an auxiliary switch 120 and a capacitor 125 coupling in parallel with two ends of a primary winding 132. The primary winding 132 is coupled in series with a power switch 110. The auxiliary switch 120 has duty time series staggered with the power switch 110. The auxiliary switch 120 includes a body diode 122 and a body capacitor 123. The body diode 122 allows current of a leakage inductance 136 to pass through to the capacitor 125. By means of the body diode 122, body capacitor 123 and the capacitor 125 coupling in parallel with the auxiliary switch 120 it can be a substitute of the conventional passive snubber to provide functions as desired. Moreover, by synchronizing a control signal of a control means 150 with operation of the power switch 110 (at opposite duty time series) an active snubber can be formed. However, due to rapid charging and discharging a higher current IA is generated to pass through the auxiliary switch 120. The higher current causes a greater conductive loss and results in other problems because of the loss. Furthermore, resonance is generated between the capacitor 125 and anther capacitor 142 at the secondary side of the transformer that causes shifting of current phase at the secondary side. As a result the switches at the secondary side endure a higher spike voltage and result in a greater switching loss and generation of noise during switching. In order to overcome the aforesaid problems, another U.S. Pat. No. 6,069,803 entitled “Offset resonance zero volt switching flyback converter” discloses a technique to correct the problem of current phase shifting at the secondary side. In short, the passive snubbers proposed in all the techniques previously discussed are not being controlled to provide synchronous function, while the active snubber generates excessive current and causes current phase shifting at the secondary side. Although the problem of current phase shifting at the secondary side can be resolved through another circuit, the circuitry becomes more complex and the cost is higher, and circuit loss also increases.